It is known that combustion by compressed self-ignition in which a mixture gas combusts instantly without flame propagation maximizes fuel efficiency since the combustion period is the minimized. However, various problems must be solved for automobile engines with regard to combustion by compressed self-ignition. For example, since operating states and environmental conditions vary greatly in the automotive application, performing compressed self-ignition in a stable manner is a major problem. Combustion by compressed self-ignition has not yet been put to practical use for the automobile engine. In order to solve the problem, for example, JP4,082,292B2 proposes that an ignition plug ignite the mixture gas, when it is difficult for the compressed self-ignition to occur because of a low combustion-chamber temperature. By igniting the mixture gas immediately before a compression top dead center, the pressure around the ignition plug increases to facilitate the compressed self-ignition.
Unlike the technology disclosed in JP4,082,292B2 in which the compressed self-ignition is assisted by the ignition of the ignition plug, the present applicant instead proposes SPCCI (SPark Controlled Compression-ignition) combustion which is a combination of SI (Spark Ignition) combustion and CI (Compression-ignition) combustion. SI combustion is combustion accompanied by the flame propagation initiated by forcibly igniting the mixture gas inside the combustion chamber. CI combustion is combustion initiated by the mixture gas inside the combustion chamber carrying out the compressed self-ignition. SPCCI combustion is combustion in which, when the mixture gas inside the combustion chamber is forcibly ignited to start the combustion by flame propagation, the unburnt mixture gas inside the combustion chamber combusts by the compression-ignition because of a pressure buildup due to the heat generation and the flame propagation of the SI combustion. Since SPCCI combustion includes CI combustion, it is one form of “combustion by compression-ignition.”
CI combustion takes place, when the in-cylinder temperature reaches an ignition temperature defined by the composition of the mixture gas. Fuel efficiency can be maximized, if the in-cylinder temperature reaches the ignition temperature near a compression top dead center and CI combustion takes place. The in-cylinder temperature increases according to the increase in the in-cylinder pressure. The in-cylinder pressure in SPCCI combustion is a result of two pressure buildups: a pressure buildup by the compression work of a piston in a compression stroke, and a pressure buildup caused by the heat generation of the SI combustion.
Here, the compression work of the piston is defined by an effective compression ratio. If the effective compression ratio is too low, the pressure buildup by the compression work of the piston is small. In this case, unless the flame propagation in SPCCI combustion progresses and the pressure buildup caused by the heat generation of the SI combustion increases considerably, the in-cylinder temperature cannot be raised to the ignition temperature. As a result, since a small amount of mixture gas is ignited by the compressed self-ignition, and a large amount of mixture gas combusts by the flame propagation, the combustion period becomes longer and fuel efficiency decreases. That is, in order to stabilize the CI combustion in the SPCCI combustion to maximize fuel efficiency, it is necessary to have the effective compression ratio above a certain value.
Meanwhile, if CI combustion takes place near the compression top dead center because of a high in-cylinder temperature at a compression starting timing due to a high ambient temperature, etc., the in-cylinder pressure excessively increases to create excessive combustion noise. In this case, the combustion noise can be reduced if the ignition timing is retarded. However, if the ignition timing is retarded, since the CI combustion takes place when the piston falls considerably in an expansion stroke, fuel efficiency is lowered. Since the pressure buildup caused by the heat generation of the SI combustion can be utilized in the SPCCI combustion, it is effective to lower the effective compression ratio to reduce the pressure buildup by the compression work of the piston in order to achieve both reduced combustion noise and improved fuel efficiency. Thus, combustion noise can be kept to a tolerable level without lowering fuel efficiency.
In order for a design engineer to put to practical use an engine which performs the SPCCI combustion, he/she needs to determine, depending on each engine operating state, the minimum effective compression ratio at which the CI combustion is stabilized, and additionally needs to raise the effective compression ratio within a permissible combustion noise range and determine the effective compression ratio without the combustion noise becoming excessively large. Therefore, the design engineer can put to practical use the engine with maximum fuel efficiency by maximizing the ratio of the CI combustion within the SPCCI combustion, while suppressing the combustion noise to a tolerable level.
However, since the SPCCI combustion is a new combustion system in the art, no one has found the suitable range for the effective compression ratio until now.
The effective compression ratio of the engine is determined based on a geometric compression ratio and a close timing of an intake valve. Since the intake valve close timing is changed when the engine is actually operated, when implementing engine control logic for performing the SPCCI combustion, the design engineer needs to determine the intake valve close timing.
Since the maximum in-cylinder pressure increases as the geometric compression ratio increases, the strength of engine components needs to be raised, which results in an increase in weight, and an increase in mechanical resistance loss. Meanwhile, in terms of thermal efficiency, a larger expansion ratio which is determined based on the geometric compression ratio is desirable. Since the maximum in-cylinder pressure varies by the heat balance based on the combustion mode or the stroke capacity even if the geometric compression ratio is same, the optimal geometric compression ratio for the new combustion system of SPCCI combustion has been unknown until now.